This invention relates to gas flow rate monitors employing hot wire anemometer sensors, and more particularly to such a monitor enabling a fast, accurate and reliable means of calibrating each hot wire sensor.
Gas flow rate monitors equipped with hot wire anemometer sensors are especially suitable for use in making continuous measurements of gas flow rate in the breathing system of an anesthetized and paralyzed patient whose breathing must be assisted by ventilator machinery such as those described in U.S. Pat. No. 4,637,385 issued Jan. 20, 1987 and U.S. Pat. No. 4,581,945 issued Apr. 15, 1986. From a gas flow rate measurement, other important information such as tidal volume and minute volume may be calculated. The hot wire anemometer for this critical purpose is much preferred over other known sensors such as the rotary vane sensor, the Pitot tube, differential pressure drop sensor across a laminar flow element or across other gas-flow resistance elements such as wire screen, ultrasonic sensors, heat transfer sensors, and thermistors.
Hot wire sensors, usually employing platinum wires and the associated instrumentation are capable of stable reliable operation when subjected to the rough handling and harsh environment to which pulmonary gas flow monitors are typically subjected. Hot wire sensors for use in measuring the flow of a patient's breath are preferably made by mounting the wires in a cylindrical light-transparent tube so that it can be readily observed when blood, vomit or any other body fluids that pass through the tub indicating what trouble the patient may have, and alerting the doctor that such body fluids may collect on the platinum wire throwing the sensor out of calibration.
A gas flow monitor with hot-wire sensor is described by Rusz et al in the U.S. Pat. No. 3,913,379 issued Oct. 21, 1975. This gas flow monitor has a resistor bridge with two adjacent branches that consist respectively of a remotely connected pair of platinum wires that are mounted in the tube of the hot-wire sensor. Calibration of each hot wire sensor is accomplished by changing the balance of the bridge using a high resistance potentiometer connected across one branch of the bridge to obtain under zero gas flow conditions a bridge output voltage of a predetermined value.
That setting of the potentiometer is to remain fixed during ensuing flow measurements that are made using that particular sensor. Hot wire sensors vary from one to the other and each must be individually calibrated before use.
During the calibration procedure it is important that the cylindrical tube of the sensor be blocked at least at one end so that no air may flow through the tube. Also during the calibration it is necessary that the tube remain horizontal, because otherwise the hot platinum wire will cause convective air currents within the tube leading to an erroneous calibration of the zero-air-flow state.
The later two conditions for assuring zero air flow during calibration are particularly important since the sensitivity of hot wire sensors is greatest for zero flow and dimishes substantially as air flow increases. Thus it is customary to manually hold the sensor tube about horizontal while plugging one or both tube ends. This manual handling is clumsy and tends to be done only approximately adversely affecting the calibration accuracy.
It is therefore an object of this invention to provide an improved hot wire gas flow sensor capable of being easily blocked and oriented properly during calibration.
It is a further object of this invention to provide a gas flow monitor enabling a simpler more reliable procedure for accurate calibration of hot-wire sensors.